According to USA.gov, those seeking naturalization as a U.S. citizen must be at least 18 when they file an application for naturalization (Form N-400). An applicant must be a lawfully admitted permanent resident of the U.S. and must have been a permanent resident for at least five years, or for at least three years if the applicant meets all eligibility requirements to file as a spouse of a U.S. citizen.
Applicants must demonstrate continuous permanent residence and a physical presence in the U.S. and must have lived in the states or a U.S. Citizenship and Immigration Services jurisdiction. They also must demonstrate good moral character and an attachment to the principles and ideals of the U.S. Constitution.
Applicants must demonstrate an ability to read, write, speak and understand basic English, as well as have a basic knowledge of U.S. history, government and civic principles. Once a naturalization application is approved, the applicant must take the Oath of Allegiance to the U.S. before receiving their naturalization certificate.